on a mac how do I get rid of exclamation points?

I have 17,000 (actually 35,483 - many of the songs have duplicated themselves when I was attempting to do a "copy/backup" of my music library to an external harddrive) songs that show exclamation points next to each song. How do I get ride of them - in bulk. Individually would be a bit much. Also, how do I get rid of the duplicates?

Thanks for your help.